Organic thin film solar cells are solar cell elements using organic matter for an active layer and as a charge-transporting substance, among which the dye-sensitized solar cell developed by M. Gratzel and the organic thin film solar cell developed by C. W. Tang are well known (Non-patent Documents 1 and 2).
Both types of solar cells have different characteristic properties from those of the inorganic solar cells which are the main stream at present, in such points that they are light in weight, are thin films, can be made to be flexible, and can be produced on a roll-to-roll basis, so that they are expected to form a new market.
Among others, the organic thin film solar cells (organic photovoltaics; hereinafter abbreviated to OPV) have been drawing great attention, since they are characterized in that they are at least free of electrolytes and free of heavy metal compounds, and since a photoelectric conversion efficiency (hereinafter abbreviated to PCE) of 10.6% has recently been reported by a group of UCLA et al. (Non-patent Document 3).
In recent years, utilization of renewable energy has been advocated, and developments thereof have been being accelerated. In regard of the OPV, investigations have been under way for putting devices of energy harvesting and the like into practical use in their early stages. Besides, the following further wider ranges of use of the OPV devices are expected.
However, PEDOT/PSS often used as an anode buffer layer in the OPV is prepared as an aqueous dispersion, and it is very difficult to remove water completely or to inhibit reabsorption of water; therefore, there is a problem that water is liable to accelerate deterioration of the OPV element. In addition, since the PEDOT/PSS aqueous dispersion liable to show aggregation of solids, there are problems that defects are liable to be generated in the coating film, that the coating apparatus is susceptible to clogging or corrosion, etc.; thus, there have been problems yet to be solved, also in the mass production process.